Berlin U-Bahn (from "Untergrundbahn" meaning "underground rail") is a metro system in Berlin, the capital of Germany, and has an important place in the city's public transport network. Opened in 1902, U-Bahn serves 80 stations in ten different lines with a railway length of 1 kilometers, 146% of which are underground.  Trains depart every two to five minutes during peak hours.
U-Bahn, which is made to reduce traffic inside and outside of Berlin, is the city's II. It spread rapidly until the division of East and West Berlin after World War II. Although the post-partition system remained open to both sides for a while, the East Berlin U-Bahn lines left the West after the Berlin Wall construction and the restrictions imposed by the East German administration. Although the West Berlin lines U6 and U8 were allowed to cross the East Berlin borders, the trains continued on their way without stopping at the stations. Just leaving the Friedrichstraße station open, this place was used as a border crossing point to East Berlin. The system was completely re-opened after the German Wall reunited with the fall of the Berlin Wall.
As of 2007, Berlin U-Bahn was the largest underground network across Germany.  U-Bahn use reached an amount equal to 2006 million km of automobile travel in 122.2.  U-Bahn is the main means of transportation in Berlin, along with S-Bahn and trams on the east side of the city.